This invention relates generally to keying means for preventing rotation of one steam turbine wheel relative to an adjacent wheel, both wheels being carried by a shaft, in the event a shrink fit between the one wheel and the shaft loosens.
Some steam turbines utilize such large rotors that the turbine wheels, which carry the turbine blades at their radially outermost portions, are not an integral part of the shaft of the steam turbine rotor. The radial dimensions of such turbine rotors may be on the order of seven or eight feet excluding turbine blade dimensions. It is well known in the art that such large rotors are subjected to substantial stresses due to their size, and to quality and quantity of steam affecting their turbine blades.
Each turbine wheel includes a hub section generally at its radially inner portion of the wheel and each hub section includes a bore therethrough. The wheel, which is not an integral part of the shaft, is typically secured to the shaft by an interference shrink fit between the radially inner surface of the hub defining the wheel bore and corresponding surface of the shaft. During normal and expected turbine operations, this interference shrink fit prevents rotation of the turbine wheel relative to the shaft and relative to other wheels secured to the shaft.
To ensure proper operation of the turbine, it is required that turbine wheels be maintained at a substantially fixed radial position, relative to the shaft, and at a substantially axially fixed position relative to other wheels on the shaft. These requirements should be met during all turbine operations, including normal but non-steady state conditions, such as overspeed while setting trip limits, and during undesirable abnormal conditions, such as overspeed due to control malfunction or runaway, and during thermal transient periods.
It is recognized in the art that the wheel bore and shaft surface interface is under a substantial degree of stress. This stress, in combination with other stresses such as generated by transient thermal conditions or other unavoidable adverse operating conditions, has been suspected of causing stress corrosion cracking indications in certain regions of the hub of wheels. The precise mechanism which produces stress corrosion cracking is not fully understood, however, it is believed that if stresses at the wheel bore, and along the hub inner surface, are kept at a minimum, and accumulation of water, which may be condensed from steam, is minimized and/or eliminated in that region, the probability of stress corrosion cracking developing in any particular wheel will be reduced if not eliminated.
U.S. Pat. No. 4,029,437, by Aubry et al., discloses a button disposed in a radially intermediate keyway defined between adjacent hubs of adjacent steam turbine wheels. Other disclosures show cylindrical buttons set into keyways to prevent rotation of one wheel with respect to an adjacent wheel in the event the interference shrink fit loosens between the one wheel and the shaft. However, these disclosures do not specifically address local stress in the region proximate the keyway, nor do these disclosures address the shape of the key.